Gun Mount

When I started with this part, I had something steampunkish on my mind. It was almost free-form modelling. First I created 16-sided cylinder, made some Insets, Bevels / Extrudes and some very small Chamfers to make the metal part (1). Then I made Extrude along spline and added some big Chamfers to make that rubber like hose (2). The object ends with simple shape which should be an electric engine (3). Finally I added some low poly spheres with Array tool to the hard metal part, to make the bolts. The metal plates are simple boxes with some cuts and small Chamfers.

Minigun

I simply love that type of gun, I could put it in every military design I make :) There are tons of pictures of miniguns on the internet, so you can choose whatever you like. Although a minigun is just a bunch of cylinders, remember to put some details on it, as it won't look good without them. For example insets on the end of the barrel, or simple rings.

Ammo

I made one bulet using .50 Browning as a general reference (not exactly following it). The bullet and the chain link are one object. I instanced it over a spline which led from the ammo box to the minigun and manually rotated them a bit for a better connection.